While there are several compounds which contribute to the greenhouse effect, carbon dioxide (CO2) has received the most attention, due to its abundance as an effluent in industrial processes. Therefore, the literature has shown a concentration on developing a separation scheme which can efficiently and economically capture and recover the effluent CO2. While the present state of the art for CO2 removal allows for such a process to be applied, the economics of the process are not favourable enough to offset the capture cost. The major obstacle to these processes is the dynamic efficiency of the separation medium being employed; membrane, absorbent, or adsorbent. The most common method of CO2 removal presently used on a large scale is via wet scrubbing (liquid phase absorption).
The use of gas scrubbing processes for environmental protection or for manufacturing of chemicals is widespread in industry (Kohl et al., 1997). Removal of various gaseous pollutants such as volatile organic compounds (VOC), NOx, SOx, HF, HCl, H2S, CO2, phosphine and arsine often takes place via wet scrubbing, typically in counter-current towers using either pure solvents (e.g., water or oil) or solvents containing dissolved materials which may consist of bases (Thomas and Vanderschuren, 2000; Bai Yeh, 1997), salts (Lynn et al., 1996) or oxidants (Overcamp, 1999; U.S. Pat. No. 5,527,517 (1996); Chien and Chu, 2000). There are also “semi-dry” scrubbing processes using a slurry of solid particles which react with targeted species. in the gas phase, ideally in a spray tower (Eden and Luckas, 1998). Dry scrubbing of gaseous acids using finely divided solid sorbents, such as calcium oxide, hydroxide or carbonate in a cyclone reactor was also found, at the laboratory scale, to be highly efficient, particularly when partial recirculation of the solid reactant is achieved (Fonseca et al., 2001).
Carbon dioxide scrubbing is currently used on a large scale for the purification of industrial gases, for example, in natural gas processing and potentially the fuel cell industries. Carbon dioxide is also removed in life support systems in confined space (submarines, space shuttle and other inhabited engines for space exploration). These processes mainly use alkanolamine aqueous solutions (Astarita et al., 1983), the most common being mono- and diethanolamines, (MEA and DEA) and N-methyldiethanolamine (MDEA). The process is reversible, with the formation of carbamate and bicarbonate favoured at low temperature and their dissociation to amine and CO2 favoured at a slightly higher temperature. To maximise the CO2 adsorption capacity, it is therefore important to either enhance the hydrolysis of carbamate or limit its formation.
The use of aqueous solutions of low molecular weight alkanolamines suffers a number of drawbacks (Hook, 1997; Veawab et al., 1999). Under scrubbing conditions, (i) a fraction of the amine and its decomposition products are lost by evaporation, which, in addition to reducing the absorption capacity, may cause problems because of their toxicity, (ii) the amine undergoes oxidative degradation leading to decreased capacity, increased viscosity and excessive foaming, (iii) excessive corrosion takes place, thus posing severe operational problems.
Introduction in the mid-eighties of the so-called sterically hindered amines by Exxon (Sartori and Savage, 1983) mitigated these problems to a great extent. However, such hindered amines exhibit lower rates of CO2 absorption. The use of high-efficiency column internals such as structural packing, or high surface area membranes leads to improved mass transfer coefficients which compensate, at least partly, for the intrinsic low reactivity.
More recently, research has focused on regenerable gas-solid adsorption as an alternative separation technique. Various zeolites and other porous materials have been examined, however many of the adsorbents developed thus far suffer from problems such as low capacity, poor selectivity, poor tolerance to water, and high temperature regeneration or activation. An example of a commercialized adsorbent for CO2 removal from gas streams is zeolite 13X. When used for CO2 separation, this adsorbent requires very stringent moisture control in the inlet gas stream due to its high affinity and adsorption capacity for water. When exposed to water, the material should be regenerated at temperatures between 300° C. and 400° C. in order to recover its high CO2 adsorption capacity.
Solid Supported Amines: The idea of combining amines with solid supports to afford CO2 adsorbents has been examined by several groups as discussed hereafter. The materials were prepared either by grafting of amine containing alkoxy-silanes onto the surface of the support or by deposition of amine containing molecules onto the support. Common problems encountered when developing amine loaded solid supports were low capacity for CO2 due the limited quantity of amine retained on the support, operation in a narrow temperature range, and poor thermal stability. The rate and capacity of CO2 adsorption on such adsorbents depend chiefly on the amine loading and the porosity of the material, which are not completely independent since higher amine loadings may be obtained with higher pore volumes.
In the following text, all references to materials on a per gram of support basis (gsup) is inferred as a per gram of non-functionalized material, i.e., the support material alone, whereas the reference to a per gram adsorbent basis (gads) is inferred as a per gram of support plus the added functionalization compound(s), i.e., per gram of total adsorbent mass.
Functionalization by Impregnation: Examples of solid materials with impregnated amines are provided in U.S. Pat. Nos. 2,818,323 (1957) and 3,491,031 (1970). In each case, however, these supports are characterized by low adsorption capacity and/or difficulty in regeneration. Similarly, U.S. Pat. No. 4,810,266 (1989) discloses a material for adsorbing CO2, wherein the material is a form of carbon molecular sieve containing a dihydric alcohol amine compound. The materials disclosed exhibited an adsorption capacity of only 2-2.6 wt % (0.46-0.59 mmol/gads) when exposed to a 0.4% CO2 mixture in N2, ca. 6.0 wt % (1.36 mmol/gads) for a 5% CO2 mixture, and 6.45 wt % (1.47 mmol/gads) for a 50% CO2 mixture.
U.S. Pat. No. 4,999,175 (1991) discloses a method and application for separating sulphur compounds using a support material, such as silica, alumina, clay minerals, zeolites or mixtures thereof, having an amine coating. The support material has been characterized by an active amine content of only 5-8 mmol/gsup of support, where the amine is from the group of monoethyleneamine, diethanolamine, and ethylenediamine.
U.S. Pat. No. 5,876,488 (1992) discloses a material, method and application for amine impregnated within an acrylic ester resin porous support, where the preferred amine is DEA and is loaded to the level of 53 wt % (5.05 mmol/gads). The patent indicates that the support material has a surface area of 50-1000 m2/g and an amine content between 35-75 wt % of dry support mass.
Satyapal et al. (2001) describe the use of a material containing amine functionality within the pores of a polymeric resin. The CO2 adsorption capacity of the material for a 2% CO2/N2 feed mixture was reported as 4 wt % gain (0.91 mmol/gads) and as high as 8 wt % (1.82 mmol/gads).
Xu et al. (2002, 2003) describe a mesoporous silica support, MCM-41, impregnated with polyethylene-imine (PEI). Theses studies were conducted using various loading ratios up to a maximum of 75 wt % (PEI+support), which corresponds to an over-saturated pore. The MCM-41 support material used exhibited typical characteristics of a MCM-41 type silica, namely, a pore volume of 1.0 cc/g, pore diameter of 2.75 nm, and a surface area of 1480 m2/g. With these material characteristics, an amine loading of about 1.0 cc/g could be attained at pore saturation, under ideal packing conditions. In terms of CO2 adsorption capacity, it was reported that a maximum 13.3 wt % increase (3.02 mmol/gads) was obtained when exposed to 100% CO2 at 75° C.
The PEI-MCM-41 presented by Xu et al. (2002, 2003) requires an adsorption temperature of 75° C., and a regeneration temperature of 100° C. Within this narrow thermal window, the adsorption and desorption process occurs. At lower temperatures, the material will still adsorb CO2, however, due the viscous nature of the PEI, the rate of adsorption is unacceptably low. If the temperature is increased above 100° C., then unacceptable loss of the impregnated PEI occurs.
U.S. Pat. No. 6,547,854 (2003) discloses a method of immobilizing an amine compound on a solid oxide support. The impregnation method disclosed is a multi-step, multi-component process that is time consuming. Further, the pure CO2 adsorption capacities are described, at best, reported as 0.77 wt % gain (0.175 mmol/gads).
U.S. Pat. No. 6,670,304 (2003) discloses a method for preparing an amine impregnated activated carbon molecular sieve and a use of this material as a water and CO2 adsorbent. The disclosed support material is characterized by a pore diameter of 0.5-1.2 nm, pore volume of 1.5-2.5 cc/g, and a surface area of 2000-2500 m2/g. A 0.5% CO2 adsorption capacity of 4-6 wt % (0.91-1.37 mmol/gads) is described for the temperature range of 15-25° C.
Contarini et al. (2003), and Ital. Pat. ITMI20020536 (2003) describe the impregnation of various alkanolamines and polyamines within the porous structure of silica, silica-zirconia, alumina, and clay supports. The most favourable results were apparently obtained with an alumina support of the following characteristics, pore volume of 1.1 cc/g, median pore diameter of 10.5 nm, and a surface area of 230 m2/g, impregnated to pore saturation with a 50-50% mixture of DEA and N,N′-bis(2-hydroxyethyl)ethylenediamine. For this material a 9.6 wt % (2.18 mmol/gads) increase was obtained at equilibrium with 100% CO2, and exhibited a total organic content of about 50 wt %. The disclosed material also did not demonstrate favourable desorption properties.
Zhou et al. (2004), and Chinese Patent 02117914 (2003) relate to the impregnation of triethanolamine within the pores and on the surface of a type of silica gel. The target separation is for the removal of H2S from methane. Silica gel with a pore volume of 0.85 cc/g, pore diameter of 10 nm, and a surface area of 335 m2/g was used as the support material.
Functionalization by Post-Synthesis Grafting: Feng, et al. (1997), and U.S. Pat. Nos. 6,326,326 (2001), 6,531,224 (2003), 6,733,835 (2004), 6,846,554 (2005) describe a method to produce a uniform monolayer of functionalized silane on a mesoporous support. Specifically, water was used to wet the entire surface area of the support material thereby facilitating the formation of a complete monolayer. Further disclosed was the application of post-grafting distillation to remove the produced alkanol and water azeotrope and, thus drive the silane reaction to completion. This approach was applied to a mesoporous silica characterized by a pore diameter of 5.5 nm and surface area of 900 m2/g. The grafting was carried out under an inert atmosphere, with toluene as the solvent, and at a temperature of 110° C., under reflux. This material was functionalized with a mercapto-silane compound and was used for the removal of heavy metals from water.
Chuang et al. (2003) studied the adsorption mechanism of CO2 when interacting with a primary amine site grafted on a so-called SBA-15 silica. The SBA-15 material was characterized by a low surface area of 200-230 m2/g. The performance of the material when subjected to a 4% CO2/He gas mixture was reported as 1.76 wt % increase (0.40 mmol/gads).
Leal et al. (1995), and U.S. Pat. No. 5,087,597 (1992) disclose an amino-silane functionalized silica gel and its application to the separation of CO2 from air in confined spaces. The support material was characterized by a pore diameter of 6 to 18 nm, pore volume of 0.4 to 0.8 cc/g, and a surface area of between 120 and 240 m2/g. The material was described as having an adsorption capacity of between 1.47 and 2.30 wt % gain (0.33-0.52 mmol/gads) when exposed to a dry, pure CO2 environment. The patent also discloses the method to produce the functionalized material.
Huang et al. (2003) demonstrate relatively high CO2 adsorption capacities for an amine grafted material. They examined the effect of functionalizing MCM-48 and silica xerogel with aminopropyltriethoxysilane. The MCM-48 support material was characterized by a surface area of 1389 m2/g, and the xerogel was characterized by a surface area of 816 m2/g. The aminopropyltriethoxysilane grafting was performed with toluene as the solvent and an inert gas head space, and in the absence of water at 70° C. for 18 hours, with reflux. The resulting amine grafted quantities were 1.7 mmol/gads (9.9 wt %) for the xerogel, and 2.3 mmol/gads (13.3 wt %) for the MCM-48, materials based on the propylamine chain. The corresponding 5% CO2/N2 adsorption capacities were determined as 1.14 mmol/gads (5.0 wt %) for the amino-MCM-48, and 0.45 mmol/gads (1.96 wt %) for the amino-xerogel.
There remains a need, however, for improved, regenerable materials with high adsorption capacity and rate, and with tolerance to moisture.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.